The present invention relates to electrical coil constructions and, more particularly, to spiral type coil constructions which may be directly wound on a suitable bobbin or fabricated on a substrate in a continuous strip form and provide substantially improved voltage gradient characteristics. This invention is related to the invention disclosed in applicant's application Ser. No. 384,469, filed Aug. 1, 1973, now abandoned in favor of applicant's copending application Ser. No. 549,717, filed Feb. 13, 1975 for "Ignition Coil" now abandoned in favor of application Ser. No. 894,449, filed Apr. 7, 1978, now U.S. Pat. No. 4,514,712 which is a continuation thereof.
Conventional ignition coil generally comprise a core material capable of conducting magnetic flux, such as a soft iron or other similar material, a primary winding and secondary winding, each of which is disposed over the core material. These coils are generally constructed with the secondary winding formed by wrapping successive helical layers of an electrical conductor over the core material or other forming structure until the desired number of turns is established. Typically, each helical layer of such a construction will consist of several turns of the electrical conductor laid side by side extending longitudinally along the core material with the next layer beginning at the opposite end and travelling longitudinally back over the first layer. The electrical conductor normally used is commonly referred to as magnet wire and is a small gauge copper wire generally insulated with a coating of enamel or other like material thereon. In operation, each turn of the secondary coil winding will have induced in it a voltage produced by the changing magnetic field which links that turn and which is generated by changes in the current flowing in the primary winding. This magnetic field will induce approximately an equal amount of voltage in each successive turn of the winding, but as the individual turns are all serially connected, the voltage of each turn will be added to that induced in each preceding turn. Thus, it becomes apparent that while the turn-to-turn voltage gradient within the coil may be small, as the total number of turns within each layer increases, the layer-to-layer voltage gradient, being composed of the sum of the turn-to-turn voltage gradients within each layer of two adajcent radially disposed layers, will be of a considerable magnitude. This is particularly true when successive layers are wound with alternating longitudinal travel, that is, the first layer is wound with successive turns travelling from right to left with the next layer having successive turns travelling longitudinally from left to right. In this construction, the layer-to-layer voltage at the beginning end of the winding will be the sum of the turn-to-turn gradients for two complete layers of winding.
In order to prevent these large voltage gradients from breaking down the enamel or like insulation and thereby shorting out entire layers of windings, it is common practice to introduce additional interlayer insulation. The dielectric constant of these materials in combination with the relatively large surface area of the thousands of turns of secondary winding typically found in a high voltage winding produces large distributed capacitance. This distributed capacitance must be charged each time the winding is energized and discharged each time the winding is de-energized, thus introducing substantial limitations on the speed with which this energization and de-energization may be accomplished. The addition of this interlayer insulation increases the bulk of the coil itself further resulting in increased cost of the final product.
The development of new ferrite compositions, which may be formed into structures, commonly referred to as "pot" cores, which allow a much greater rate of change of magnetic flux therein, have afforded opportunity to develop high voltage windings having only hundreds of turns instead of the thousands of turns required when they are wound on cores usually associated with conventional high voltage coil construction. These "pot" cores provide a relatively "tight" closed magnetic circuit and are readily capable of achieving an inductance of 1 Henry/1000 turns. With the capability of increasing the rate of change of magnetic flux, the voltage induced in each turn of the coil has been greatly increased to as high as several hundred volts per turn. Lack of reasonable and economical insulation systems now prevents widespread use of "tight" closed magnetic circuits in high voltage coils; most high voltage windings are now wound on relatively "open" magnetic cores and develop only from one to three volts per turn.
One solution to this problem has been to design a winding having spiralled turns, whereby each layer of the coil will consist of a single turn of the electrical conductor. Successive spiralled coils are then interconnected with suitable insulation provided therebetween to obtain the total number of turns for the desired coil application. While the basic concept of this form of construction solves the insulation problem such constructions have been extremely expensive to manufacture requiring successive spiral coils be separately manufactured and then interconnected. Further, these multiple connections decrease the reliability of the coil as the coil integrity will be only as good as the poorest of these connections. Should a single connection be less than perfect, it may decrease the coil output due to a high resistance therein or even become broken due to vibrations encountered by the coil during its use.
The present invention provides a solution to these problems through unique coil constructions employing a continuous electrical conductor which forms a plurality of axially spaced spiral windings on particular segments of a dielectric medium with other segments interspersed betwewen the axially spaced winding so as to provide insulation both for the conductor interconnecting the spiral windings and for the axially spaced winding sections.
In one form the construction may be fabricated on a continuous flexible strip type dielectric medium by forming spiral windings on a plurality of winding segments thereof and providing insulating segments therebetween. The strip thus formed of the above described alternating segments may then be folded accordion style, thus forming a completed coil assembly which may then be fitted to a core along with a primary winding, thus completing a transformer.
In another embodiment, the spiral coil may be wound directly upon bobbin fabricated from a plurality of nested washer-like elements. The use of this bobbin structure allows fabrication of a spirally wound coil using conventional coil winding machinery.
These spiral coil constructions thus provide means by which a high voltage coil may be fabricated at a cost significantly less than conventional high voltage coil constructions, while also providing a vastly improved insulation system which substantially eliminates the possiblity of the insulation system breaking down due to a high layer-to-layer voltage gradient, particularly such as those encountered when such core materials are ferrite formed into "pot" cores, offering the faster rate of change of magnetic flux, are employed. A further advantage offered by the spiral windings of the present invention is that the number of turns in an ignition coil may be reduced by a factor of almost 100. For example, a 100 turn strip winding of #38 wire in a configuration capable of developing 200 volts per turn has a resistance of 17 Ohms. This contrasts with the hundred of Ohms resistance of conventional ignition coil secondary windings. Thus, a substantially greater fraction of ignition system input energy may be delivered to the spark plug gap rather than being lost in the resistance of the secondary winding. Further, the reduced secondary winding distributed capacitance mentioned previously is of great advantage in ignition coil applications as it permits faster rise time of the secondary voltage. Secondary voltage rise times to twenty kilovolts have been observed faster than two tenths of a microsecond and the limitation in this test was likely oscilloscope probe capacitance. As is well known, fast secondary voltage rise time is a measure of the capability of an ignition system to fire fouled, wet, or flooded spark plugs.
The spiral winding constructions of the present invention may be easily impregnated and encapsulated with an epoxy composition material so as to provide additional insulation therein and a protective layer therearound. Further, should it be desirable, this impregnation and encapsulation process may also be adapted to provide a molded structure ideally suited for accommodating a ferrite "pot" core. This combination may then be further encapsulated with additional structure to form a unitary ignition coil for spark ignited internal combustion engines which is adapted to mount on and has provisions for effecting an insulated high voltage electrical connection to a spark plug. Such a structure which may be adapted for use with this strip coil is disclosed in my copending application Ser. No. 549,717, entitled Ignition Coil and filed on Feb. 13, 1975.
Other advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings.